Usage of optical spectroscopy techniques for analytical purposes is as such known from the prior art. For example WO 02/057 758 A1 shows spectroscopic analysis apparatus for in vivo non-invasive spectroscopic analysis of the composition of blood flowing through a capillary vessel of a patient. Here, the position of a distinct capillary vessel is determined by an imaging system in order to identify a region of interest to which an excitation beam for the spectroscopic analysis has to be directed. Typically, imaging as well as the spectroscopic analysis both make use of a common microscope objective enabling imaging of a capillary vessel on the one hand and allowing for focusing of a near infrared (NIR) laser beam into the skin for exciting a Raman spectrum on the other hand. Moreover, the same microscope objective is used for collection of the scattered radiation evolving from the Raman process.
In particular, by making use of a high-numerical-aperture objective and a pinhole in front of the detector, Raman spectra can be taken from a small, confocal detection volume. However, capillary vessels in the skin being detected by the imaging system can either be located on the optical axis of the microscope objective or can be located off axis. In the latter case and due to the fact, that the confocal detection volume of the spectroscopic analysis system is rather small, the Raman detection volume has to be shifted into a selected volume of interest covering at least a part of a capillary vessel that is typically located at a certain depth underneath the surface of the skin of a patient.
In order to obtain an appreciable spectroscopic signal not only the excitation beam has to be properly directed or focused into a selected capillary vessel but also spectrally shifted return radiation has to be appropriately collected and efficiently provided to a spectroscopic unit, e.g. a spectrometer.
In typical confocal arrangements the focal spot of the excitation beam features a diameter in the range of a few micrometers and the corresponding focal spot of the collected return radiation might be enlarged by a factor of ten. Typically, the spectroscopic unit has an entrance aperture that is several tens of micrometers in diameter. Hence, efficient spectroscopic analysis requires a substantial overlap between the confined return radiation and the entrance aperture of the spectroscopic unit of the spectroscopic system. Therefore, the confined return radiation has to be precisely aligned.
In confocal arrangements the diameter of the confined return radiation has to correspond to the dimensions of the aperture of the spectrometer. Because the size of the focal spots and/or the aperture are in the micrometer range, deviations and misalignment may easily occur due to e.g. varying environmental conditions like varying temperature and due to mechanical imperfections of any fixing or mounting means for various optical components of a spectroscopic systems. Typically, any misalignment or mismatch between the confined spectroscopic signal and the aperture of the spectrometer has to be eliminated by manually adjusting and aligning the optical paths of the spectroscopic system. Manual alignment of optical paths of a spectroscopic system is rather cumbersome and does not allow for a commercial distribution to an end consumer.
The document U.S. Pat. No. 6,352,502 B1 discloses a method and apparatus for obtaining feedback to drive a servo system for aligning and maintaining alignment in optical systems that bring light to an in vivo skin sample. This method comprises irradiating a tissue of interest in a subject with light having an excitation wavelength and that passes from a light source through a first adjustable lens, and passing spectra that are emitted by the tissue through a second adjustable lens. The spectra that are passed through the second adjustable lens are then collected and analyzed to determine a target signal associated with an analyte of interest. The method further comprises deriving a correction signal from the target signal and adjusting the position of the first adjustable lens or the second adjustable lens on the basis of the correction signal so as to enhance the target signal.
The method disclosed in U.S. Pat. No. 6,352,502 B1 derives a correction signal from a target signal, wherein the target signal is determined by analyzing collected spectra. In extreme situations where a collected spectrum does by no means enter a spectroscopic unit because of severe misalignment, no target and/or correction signal can be obtained or determined. In such extreme cases, the method and apparatus disclosed in U.S. Pat. No. 6,352,502 might not be able to appropriately enhance the target signal. Moreover, the disclosed apparatus makes use of a first and a second adjustable lens that are dedicated for directing radiation towards the sample and for collecting return radiation, respectively.
The present invention therefore aims to provide an improved alignment of return radiation collected by an optical arrangement of a spectroscopic system.